This invention relates to a method of entrenching an underwater structure such as a pipeline, and to an apparatus for forming trenches and entrenching a pipeline in accordance with the method.
When a pipeline or cable is placed on the floor of a body of water, for example, in the Gulf of Mexico, or the floor of an ocean, it is usually required that the pipeline be entrenched or buried in the floor of the body of water. Burying the pipeline or cable is desirable for various reasons, among others, to prevent damage to the line as a result of underwater currents, and to generally prevent damage to the line from passing ships where the line is located in relatively shallow water.
Typically, the pipeline is covered with a protective casing or coating of cement and/or asphalt. While it is usual to entrench a pipeline by first laying the pipeline on the bottom of the body of water and then forming a trench under the pipeline so that the pipeline gradually sinks into the trench, the pipeline or cable could be entrenched as it is fed from a pipe laying barge.
Numerous techniques and apparatus are known for entrenching submerged pipeline or cable. Among these are rotary mechanical cutters with or without high pressure fluid jets, frequently in combination with suction dredges. Typical examples of the prior art techniques and apparatus are shown and disclosed in the following U.S. Pat. Nos.:
3,583,170 PA1 3,732,700 PA1 3,732,701 PA1 4,037,422 PA1 4,117,689
Dredges and high pressure jets, while quite effective for forming underwater trenches are expensive to operate because of the surface pumps and equipment required to operate the jets and the dredges. Further, where a suction dredge is used, the material sucked out of the trench is usually carried to the surface and then dumped with the result that any polutants on the ocean floor are mixed with the water and, in any event, the water becomes and stays muddy for a considerable period of time.
The mechanical cutters used in the past have not been wholly effective for the reason that where the floor of the body of water is silty or soft, these cutters have a tendency to cause some of the silty material to float temporarily and then settle back into the trench before the pipeline bottoms in the trench. Where rocks are encountered, the mechanical cutters are quickly dulled, and even when suction dredging is also used, the suction equipment cannot remove large rocks. Water jets are normally wholly ineffective where rocks are encountered. In the case of a bottom composed of clay, the cutters have a tendency to shear off large chunks which cannot be removed effectively with dredging equipment. Trenching with water jets when clay is encountered is very slow and time consuming because of the tenacious nature of the clay.